An Enzyme-linked Immunosorbant Assay (ELISA) has been developed for testing rabies vaccine for potency. 96-well plates are first coated with serial dilutions of the International Reference Preparation and test vaccines, followed by incubation with the U.S. Reference Human Rabies Immune Globulin. Antigen concentration is determined after incubation with anti- human IgG coupled to peroxidase, a final incubation with substrate and optical density measurements on an ELISA plate reader with computer interface. Data analysis to determine vaccine potencies by comparison with the reference vaccine, as well as statistical significance of results, is performed using a statistical software package. Optimal working dilutions or concentrations of all reagents, including vaccines, immunoglobulin, peroxidase-linked secondary antibody and blocking agent, have been determined. We are continuing to perform the ELISA potency test on vaccines in conjunction with other in vitro potency tests including the Single Radial Immunodiffusion Test and the Modified Antibody Binding Test. All in vitro test results are then compared with NIH mouse potency test results. Our goal is to find a suitable in vitro test to replace the NIH test which is the current official rabies vaccine potency test.